There exist many applications in industry and science in which it is required to determine the size and number of particles suspended in a fluent (i.e., liquid or gaseous) medium. These determinations can be very important in many manufacturing processes such as pharmaceuticals, plastics, chemicals, and paper, to name a few examples. Processes such as crystal growth, precipitation, polymerization, gravimetric separation, grinding, etc., must be monitored to control the quality of the product.
The particles in a slurry can range from submicron to millimeters in size, and the relative quantities of the different sizes may be very critical to the quality and performance of the product. For example, a pulp slurry may contain fibers which are about 10-40 microns in size as well as brightener and filler particles (clay) which are mostly less than 4 microns in diameter. The size of coal dust will influence the rate at which it burns. The powders in pills and capsules must be ground to specific sizes to dispense drugs at optimum rates in the human body.
Ideally, the particle measurements should be made on-line to provide realtime information for process control, and also avoid distortion of the particle size information by removing samples from the process.
In the past, a number of different technologies have been developed for the analysis of particle size, using conventional or laser light sources, ultrasonic beams, cathode ray tubes, and Stoke's law of sedimentation rates. The optical devices which exist today utilize transmission geometries and use various methods to determine size. The method used in most commercially available instruments measures the light intensity scattered at various small forward angles (e.g., Fraunhoffer diffraction patterns) to determine particle size. Another method uses a beam of light scanned scross a small chamber through which the fluent medium is force to flow. A detector is positioned on the opposite side of the flow such that particles cause interruptions of the light received by the detector (Staffin, U.S. Pat. No. 3,676,647, and Ogle, U.S. Pat. No. 3,858,851). The time that the beam is interrupted is used as a measure of the size of the particle. A variation on this method is devices which force the particles through a sampling chamber at a known velocity and keep the beam stationary (Eisert, U.S. Pat. No. 4,110,043).
The major reason for the use of the transmission geometry is that the light scattered by particles in the forward direction (i.e., less than 90.degree. from the original direction of the light beam) is up to 10,000 times more intense than the light scattered in the backwards direction. The limitation of transmission systems is that the particle concentration in the medium must be very low (less than 0.01 percent by volume) to allow the light to pass through the fluent medium, be scattered by a particle, and then pass further through the medium to be detected. Obviously, the use of such methods is impossible at normal process concentrations, which are commonly much greater than one percent. Attempts to use such devices on-line must involve sampling systems which are susceptible to clogging, and dilution systems that cause errors by altering the composition of the sample.